The present invention relates to magnetic recording media such as magnetic tapes, magnetic discs, etc. More precisely, it relates to magnetic recording media, such as video tapes, audio tapes, etc., which have plural magnetic layers on a non-magnetic support.
Recently, means of laminating plural magnetic layers on a non-magnetic support to produce magnetic recording media, such as video tapes, audio tapes, etc., having improved characteristics have been employed with the tendency of VTR, etc. toward shortened electromagnetic waves for recording and toward increased densities of information to be recorded.
These magnetic recording media, as having plural magnetic layers each differently comprising magnetic powders having different magnetostatic characteristics, particular binders, additives, etc., have particular frequency characteristics, electromagnetic conversion characteristics, durability, etc., which, however, other magnetic recording media having a single magnetic layer could not have.
To produce such magnetic recording media having plural magnetic layers, it is necessary to develop different magnetic compositions for upper and lower magnetic layers which differently satisfy the necessary magnetic characteristics and mechanical characteristics for these layers. It is well known that coated magnetic recording media to be produced by coating a magnetic composition, which is obtained by mixing and dispersing a fine ferromagnetic powder in an organic solvent along with a binder, an abrasive and other additives, on a support have significantly different characteristics, depending on the binder used.
For instance, the uppermost magnetic layer of such a coated magnetic recording media having plural magnetic layers, which slides on a head relatively to the motion of the head, is required to have good durability. Therefore, a relatively large amount of a lubricant is added thereto, or the layer often contains a binder having a glass transition point higher than that of the binder in the lower magnetic layer by 10.degree. C. or more (see Japanese Patent Laid-Open No. 2-105326). The upper layer often contains a binder having a larger molecular weight than that of the binder in the lower layer (see Japanese Patent Laid-Open Nos. 1-79931 and 1-263927).
The lower layer often contains a polyurethane resin so as to improve the adhesiveness between the layer and the non-magnetic support to thereby improve the contact between the magnetic recording medium and the head with which the medium is kept in contact (see Japanese Patent Laid-Open No. 3-173923).
In addition to these mentioned hereinabove, other various binders for magnetic compositions for forming magnetic layers have been investigated with respect to their structures and polar groups in order to improve their dispersibility in magnetic compositions (see Japanese Patent Laid-Open Nos. 2-110825 and 3-80425).
However, if magnetic compositions comprising such known binders are coated on a support to form plural layers thereon by high-speed extrusion coating or by wet-on-wet coating where the upper layer is coated while the lower layer is still wet, streaks are often formed in the coating direction on the coated surface. Such streaks are significantly problematic in that these often worsen the degree of roughness of the surface of the magnetic recording medium having the layers and that these often cause spacing between the surface of the magnetic recording medium and the head with which the medium is kept in contact, thereby worsening the electromagnetic conversion characteristics of the medium.
The necessary characteristics of binders to be in magnetic layers differ between the uppermost layer that slides on a head relatively to the motion of the head and the other layers (lower layers). Depending on the combination of such binders, therefore, streaks are often formed in the coating direction on the coated surface of a magnetic recording medium having plural magnetic layers, as mentioned above. As the case may be, large streaks are often formed. It is known that binders having good coatability and their combinations are important. Therefore, it is necessary to investigate such binders and their combinations. In addition, binders for magnetic compositions must still be improved in order to satisfy both the coatability and the dispersibility of the magnetic compositions containing them.